It is possible to perform local heating by directing laser light at a thin film made of a chalcogen material or the like formed on a substrate, and use differences in irradiation conditions to effect a phase change between an amorphous phase and a crystal phase with different optical constants (refractive index n, extinction coefficient k). This is already well known, and rewritable digital versatile discs (DVD), Blu-ray Discs (BD), and the like have become very popular as optical information recording media of the so-called phase change type that makes use of this phenomenon.
With an optical information recording medium having a phase-change recording layer, it is possible to delete an existing signal and at the same time record a new signal by irradiating the recording layer while modulating the laser output according to the information signal between two or more power levels. In general, a phase-change recording layer is in an amorphous state at the point when it is formed by sputtering or another such means, and is crystallized (this is considered the initial state) by performing laser annealing or another type of initialization. This recording layer is irradiated with a high-power laser to heat it above its melting point, and then quenched to produce an amorphous phase (the recording state), and is irradiated with a low-power laser to raise it above its crystallization point, and then slowly cooled to produce a crystal phase.
The above-mentioned initialization requires a dedicated and expensive apparatus having a dedicated optical pickup, a focus server, and the like, and from the standpoint of mass production, it is preferable to omit this process. Also, the recording layer crystallizes and contracts after initialization, which tends to hamper stable production, particularly when the substrate is thin. To deal with this situation, Patent Literature 1 discloses that a crystallization promotion layer is provided as a substrate to the recording layer, which puts the recording layer in a crystal phase state after its film formation, and eliminates the need for initialization.